There are a variety of bicycles, which are designed to meet various purposes. The case in point is an acrobatic bicycle as shown in FIG. 1. The acrobatic bicycle has a brake lever 1 which is located under the handlebar 2. In addition, the bicycle has a U-shaped tube 3 fastened between the two handlebars 2. The handlebars 2 are provided with a tube lashing seat 4 on which an adjustment member 5 is mounted, as shown in FIG. 2. The adjustment member 5 is used to adjust the holding force of the brake lever 1. The adjustment member 5 has an action rod 5a which is connected at the front segment thereof with one end of an outer tube 7 which is intended to protect a brake cable 6. The action rod 5a is provided with a sunken hole 5b enabling the action rod 5a to engage securely with the outer tube 7. The action rod 5a is therefore extended outward to a certain extent. In view of the adjustment member 5 being contiguous to the U-shaped tube 3, the adjustment member 5 and the tube lashing seat 4 are crammed in a limited space available between the adjustment member 5 and the U-shaped tube 3. The space problem becomes acute if the handlebars 2 are shortened. In light of the space problem described above, the adjustment member 5 can not be installed and adjusted with ease and speed. The bicycles of other types also have the similar problem.